1. Field of the Invention
The present invention relates to bonding of semiconductor devices, and in particular, to tools and processes for ultrasonic ribbon bonding.
2. Related Art
In the manufacture of semiconductor devices, active elements in a semiconductor device, such as drain and/or source regions in a semiconductor die, are electrically connected to other devices or electronic components, such as on a printed circuit board. Typically, the connection is made by bonding, e.g., ultrasonically bonding, a conductive wire between the two portions. The wire can be made from gold, aluminum, or copper, with typical diameters from 12 μm to 500 μm. Examples of electrical connections which can be made using wire bonding techniques include connections between the contact surfaces of discrete or integrated chips and the contact leads of their packages, and, in the case of hybrid circuits, the connections between inserted monolithic elements and the film circuit which contains them.
A number of wire bonding techniques have been developed, and one which has been particularly successful is a micro-welding technique using ultrasound. Aluminum wire, in contact with the surface to which it is to be bonded, is moved rapidly in the direction of the surface to which it is to be bonded, so that its oxide layer breaks open. The wire is then subjected to pressure, and a permanent joint is created between the two materials. Motion of the wire is generated by an ultrasonic transducer excited by an ultrasonic generator to produce high-frequency mechanical vibrations.
One type of ultrasonic wire bonding uses a wedge bonding tool. The ultrasonic energy is directed to the aluminum wire by the wedge tool. The wire is fed through a guide at the bottom of the wedge. The wire is then pressed down with a small defined force to slightly deform the wire. Ultrasonic energy is then switched on, and the bonding process starts. During this time, the wire portion under the bond tool is deformed, primarily widened, with the actual change in shape depending on the size and the physical properties of the wire, the bond tool geometry, and the process parameter settings.
The deformation of the wire causes an extension of its surface, which is largest along the perimeter of the wire portion under the bond tool, and thus, bond formation starts there. From there the bonded area progresses inward, but typically leaves some portions of the interior unbonded or lightly bonded, i.e., the wire is not bonded completely or fully to the surface. Thus, not only must the wire deform sufficiently, but also the surface of the substrate the wire is bonded to. Because an ultrasonically created joint is based on the formation of bonds on the atomic level, an intimate material contact is a requirement for the formation of a strong bond, which itself is a requirement for a reliable bond.
In addition to wires, flexible conductive ribbons can be used to electrically connect two bonding areas. Compared to round wires, wide and thin ribbons allow bonding larger cross sections and creating larger contact areas. Ultrasonic bonding can also be used to connect the ribbon to a bonding surface. However, for the round wire's geometry, the surface extension is much more extensive with limited bond parameters (e.g., force and power) than for bonding rectangular ribbon. This makes it easier to create highly reliable bonds with round wire.
FIGS. 1A to 1D show top views of different stages of a ribbon bond using ultrasonic bonding. Ultrasonic bonding of rectangular ribbons follows a similar process and behavior to wires. In FIG. 1A, a top view of a portion of a ribbon bond 10, as for example seen after shearing off the ribbon, is shown. The tool begins to deform and bond the ribbon to the surface, as shown in FIGS. 1B and 1C, with the bond spreading inward. Typically a perimeter 12 of the ribbon bond 10 is strongly bonded, while a central area 14 of the bond is only lightly bonded, because there the surface extension is limited and exposure of clean ribbon and substrate material less likely. FIG. 1D shows a typical completed bond, with the edges more strongly bonded than the interior portions. FIG. 2 is a graph showing shear strength 20 and remaining ribbon thickness 22 as a function of (bond) time. The remaining thickness is defined here as the initial thickness minus the deformation. The four time points represent the bond at stages shown in FIGS. 1A to 1D, respectively.
The characteristics of ultrasonic ribbon bonding, discussed above, are supported by the failure behavior of such bonds under thermo-mechanical stresses caused during thermal cycling, as shown in FIGS. 3A and 3B. FIG. 3A shows a top view of a completed bond, similar to FIG. 1D above, where LB is the length of the bond and LS is the width of the strongly bonded area around the perimeter. FIG. 3B shows a side view of the bond of FIG. 3A along sectional line A-A, with the bonded ribbon still in place. Cracks 30 can start at the perimeter of the bond, where stresses caused by the mismatch between the coefficients of thermal expansion of the ribbon 34 and substrate material 36 are highest. As the joint is strong at the perimeter, the cracks are diverted from the (bond) interface into the ribbon, where their propagation is continuous but slow. After the cracks have passed this strong area, they move back to the bond interface. There they progress quickly until they meet in the center of the bond, causing a complete lift-off of the ribbon. Because of the rectangular area of the bond (wide but short), its length is the limiting dimension, being the shorter distance over which the cracks can move until they meet in the center of the bonded area.
FIG. 4 is a graph showing pull force measurements as a function of the number of thermal cycles. During a first phase 40, the strength of the heel, which is the weakest element of the bond (after bonding), especially if it had been slightly damaged due to (vertical) deformation of the ribbon, begins to degrade slowly. During a second phase 42, the bond's degradation increases, as the pull test failure changes from heel break to bond lift. This failure mode transition may occur because under thermal or power cycling the bond interface degrades quicker than the heel. When the cracks have passed the perimeter area and have moved back to the interface, the degradation rate again increases due to the low strength of the bond in this area during a third phase 44.
For ultrasonic (wire and ribbon) bonds, it is typically observed that the crack propagation rate decreases with increasing bond deformation, which is typically achieved with increasing “bond intensity” (mainly ultrasonic power, bond force). Increasing bond deformation is advantageous regarding a bond's lifetime, i.e., reliability, but in general is achieved by weakening the bond's pull strength, due to damage created to the heel.
The lifespan of a wire or ribbon bond under thermal cycling depends on the crack propagation rate and the distance the crack needs to propagate until the bond lifts off, i.e., the cracks meet somewhere in the middle of the bonded area. Consequently, the lifespan of a bond, and therefore its reliability, can be extended by increasing the distance the crack needs to propagate, by either increasing the length of the bond, and/or decreasing the crack propagation rate, i.e., by increasing the strongly bonded area towards the inner portion of a bond, and/or increasing the strength of the bond there. The latter two improvement factors require creating more locations with sufficient/significant deformation to disrupt the surface, preferably without having to increase the process parameters (force and power) significantly, and without having to severely change the shape (i.e., the aspect ratio of the cross-section of the ribbon) of the ribbon, i.e., without having to severely deform it.
Accordingly, there is a need for an improved bonding tool and process for ultrasonic bonding, which overcomes the deficiencies in the prior art as discussed above.